The proliferation of digitized media is creating a pressing need for techniques that protect copyright owners. Watermarking is a known technique for protecting copyright owners. Watermarking is a process of hiding information in digitized media for purposes of ownership verification and content protection. As expected, introduction of distortion into the host data during watermark embedding is unavoidable; nevertheless, it is usually favorable to have host data that is perceptually indistinguishable from its original form after embedding the watermark.
One well known watermarking technique is based on spread spectrum communications. In spread spectrum communications, a narrowband signal is transmitted across a wider carrier frequency band such that the signal energy present in any single frequency is undetectable. Similarly, a watermark may be embedded in host media data by spreading the embedded data over a large frequency band. Spread spectrum watermark is designed to be difficult (preferably virtually impossible) to remove from the host signal without rendering the host signal useless for its intended purposes and without imposing any perceptual artifacts introduced by the inserted watermark.
In an article by Ingemar J. Cox et al., entitled “Secured Spectrum Watermarking for Multimedia” in IEEE Transactions on Image Processing, Vol. 6, No. 12, December 1997, as well as U.S. Pat. No. 5,930,369 by Cox et al, entitled “Secure spread spectrum watermarking for multimedia data”, spread spectrum watermarking is described which embeds a pseudo-random noise sequence into digital data for watermarking purposes. Extraction of watermark requires the availability of the original image. In U.S. Pat. No. 5,848,155 by Cox et al, entitled “Spread spectrum watermark for embedded signaling”, the watermark is extracted from watermarked data without the use of an original or unwatermarked version of the data. This approach overcomes the limitations of the prior systems by using spread spectrum technology to embed watermark data or information into predetermined locations in an image.